To perform 1D simulation on single cylinder CI engine and compare it with single cylinder SI engine using GT-SUITE

Objective: To explore the tutorial of the Single Cylinder CI engine.

• Compare SI vs CI and list down important parameters
• Comment on parameters: BSFC, Exhaust Temperature, A/F Ratio
• Change MFB50 and observe the impact on performance

Given: Use the given tutorial inside GT-SUITE.

Take 1cylDI-final.gtm file from the tutorial. Save it in your working directory

Results:

                                                                                                    `tt"Comparing SI vs CI:"`

Engine Geometry:

Here, we have kept the engine geometry the same as mentioned in the tutorial. The compression ratio of the SI engine set as 9.5:1 and CI engine set as 16.5:1.

As we can see here, cylinder dimensions of the CI engine are higher as compare to the cylinder dimension of the SI engine in order to achieve a high Compression Ratio.

`tt"CASE-1"`

In this first case, we have run both engines at 1800 rpm and compare the results and noted down the following results.

1. Power and Torque:

We will see here, how much brake power and torque have obtained after running both CI and SI engines at 1800 rpm.

From the above results, we can say that if we run both SI and CI engines at the same speed CI engine has the ability to produce higher power and torque. The main reason for this is its high Compression ratio. The SI engine works on a lower compression ratio as compared to the CI engine due to its knocking phenomenon. But CI engine can work on such a high compression ratio due to its fuel property i.e. high Centane number which exhibits a fast-burning hence it requires CI principle for its operation.

   2. In-cylinder temperature, Pressure, and other important parameters:

We are going to see what will be temperature, pressure, and other important parameters inside the cylinder in both SI and CI engine.

All important in-cylinder parameters have been shown above.

1. Looking at maximum pressure values CI engine operates at very high in the cylinder pressure even at low speed compared to the SI engine because of the compression ratio and fast burning of fuel. So to withstand such high-pressure CI engine cylinder wall must be thicker. That makes diesel engines heavier, larger, and stronger than the Petrol engine. That's why generally CI engines are costly than SI engines.
2. Indicated efficiency of CI engine is greater than SI engine, more compression ratio gives more expansion ratio. Hence, the area under the PV diagram will be more in the case of the CI engine.
3. CI engines are fuel-efficient, as we can see from BSFC value above. In order to produce 1KW of power, the CI engine uses only 221.3 gm of fuel, and SI takes 239.2 gm of fuel. Also, the CI engine produces more work.
4. The air flow rate and fuel flow rate we can see is maintaining proportionately. In the SI engine we always try to work on the stoichiometric ratio (i.e. 14.5) hence, air flow rate and fuel flow rates have been maintained accordingly. In CI engine, in order to limit soot production, we always run an engine on a lean mixture (i.e. A/F ratio- 19).

    3. Emissions:

In this section we will see, what are the emissions coming out of both SI and CI engines.

Ideally, it is expected that products of complete combustion should be CO2, H2O, and N2 just passes straight through. But we can observe some pollutants at the exhaust of the engine and we have listed that above.

1. Even if the temperature inside the cylinder of the SI engine is higher than that of the CI engine, NOx formation is higher in CI engine compare to the SI engine as NOx is sensitive to temperature. The primary reason for that is the condition of the Air-Fuel mixture.
2. The SI engine we are working on is fuel Rich condition or better we will say stoichiometric condition so due to incomplete combustion, it results in carbon monoxide (CO) formation. Hence we can observe CO and CO2 values higher for the SI engine compare to the CI engine.
3. Unlike the SI engine, the CI engine operates on a Lean mixture where there is enough air to burn the fuel completely and the temperature inside the cylinder is high enough to produce harmful nitrogen gases and oxides (NOx). Hence we can observe higher NOx values for CI engine compare to the SI engine.

`tt"CASE-2"`

In this case, we are going to change the value of MFB50 (i.e. Main duration) while keeping engine speed 1800 rpm and observe its impact on engine performance.

1. First, we are going to lower the value of Main Duration with an interval of 2. (i.e. 35, 33, 31, 29)

The variation in the burn rate is shown below where we are lowering the value of the main duration.

Looking at the graphs above we can say that lowering the main duration (MFB50) causes an increase in burn rate and the variation along crank angle becomes steeper.

2. In this section, we are going to increase the main duration with an interval of 2 (i.e. 35, 37, 39, 41).

The variation of burn rate along with the crank angle is shown below.

Increasing the main duration, the burn rate curve tries to flatten as it lowers the burn rate value.

Results:

Here, we have simulated the case by decreasing and increasing the main duration, and the results are tabulated below:

From the above values the following conclusion we can draw:

• As we are increasing the burn rate by decreasing the main combustion duration. It shows a little increase in brake power output. We can see that there is a 3.58% increase in power output by decreasing the main combustion duration almost by 20.64%.
• Considering air flow rate, fuel flow rate, and BSFC values we can observe that we can have an increase in power output even without an increase in fuel consumption or in other words we can say fuel required to produce a 1 kW of power is less.

• Maximum pressure values are considerably increased so the engine should be designed such that it withstands such high-pressure during operation. To increase indicated efficiency by 3.64%, the pressure values have been increased by 9.78%.
• The higher burn rate causes higher in-cylinder temperatures. Even if the in-cylinder temperature is higher, the exhaust temperature showing its reducing trend due to less burn rate in the late combustion phase of the combustion stage.

We have increased the main combustion duration with an interval of 2 and the result we got is as above.

• As we increase the main combustion duration, we can observe from the above value that brake power and brake torque has decreased due to a decrease in burn rate.
• At the same time, we can observe that value for BSFC is increasing with increasing main duration while keeping the A-F ratio close to 19.

• Indicated efficiency, maximum pressure, maximum temperature decrease due to lowering burn rate.
• As we can see burn rate is still higher in the late combustion phase of the CI combustion stage is the main reason for increasing exhaust temperature.

`tt"CONCLUSION"`

In this way, I have compared both SI and CI engine performance by running engines at 1800 rpm and recorded some of the important performance parameters. All the detailed discussion I have done in the respective sections.
In the first section, I discussed Power & Torque, Key cylinder predictions, and emissions of both SI and CI engine where both engines are run at 1800 rpm.

In the next section, I have changed the values of the main combustion duration of CI engine running at 1800 rpm. First I increase the main combustion duration and recorded data and then I decreased the same value and listed out all important parameters.

Some of the conclusion I can draw here is as follow:

• The compression ratio of the DI engine is higher as compared to the SI engine, hence in-cylinder pressure is very high comparatively so diesel engines need to be heavier, stronger than SI engines in order to withstand such high value of pressure.
  
• Diesel engines work on lean mixture to limit exhaust temperature in order to improve durability and prolong the life of exhaust components but due to working on a lean mixture, it increases NOx formation which is harmful to human and environment hence we require various after-treatment devices in case of a Diesel engine which makes Diesel engines costly.
• Diesel engines have less BSFC compare to SI engines of same capacity which makes diesel engines fuel economic. This is because the diesel engine work on a higher compression ratio hence we will have a high expansion ratio as well which increases its thermal efficiency. Because of high power, high torque, less BSFC diesel engines are preferred for heavy-duty work like trucks, buses, rails.
• Also, because of high inertia of moving parts diesel engine works at lower speed compared to SI engines.